The proposed studies address very basic questions regarding plasticity and recovery of respiratory function following upper cervical spinal cord injury (SCI). There are about 11,000 new cases of SCI in the United States each year, with nearly 500,000 people affected. Most SCI's are incomplete with some sparing of spinal cord pathways. Among SCI patients, about 52% involve the cervical spinal cord and in many cases this results in impairment of rhythmic phrenic nerve activity and paralysis of the diaphragm muscle. Some of these SCI patients must be maintained on long-term mechanical ventilation, with associated higher morbidity and mortality rates. Clearly, it is important to understand how rhythmic phrenic activity can be restored in these SCI patients and this is a key objective of the proposed research. It is well established that excitatory premotor drive to phrenic motoneurons emanates predominantly from the ipsilateral medulla. As a result, after C2 spinal cord hemisection (SH) ipsilateral excitatory input is removed and rhythmic phrenic activity disappears on the affected side. However, there is a latent contralateral excitatory premotor input to phrenic motoneurons that can be strengthened with time after SH (neuroplasticity) leading to functional recovery of rhythmic phrenic activity. Converging evidence suggests that neurotrophins (e.g., brain- derived neurotrophic factor - BDNF) acting through tropomyosin related kinase receptors (e.g., TrkB) play an important role in neuroplasticity. Our central hypothesis is that functional recovery of rhythmic phrenic activity after SH is enhanced by an increase in TrkB.FL signaling in phrenic motoneurons. Our long-term goal is to develop an effective therapy to increase TrkB.FL expression in phrenic motoneurons and thereby promote functional recovery after upper cervical SCI. We propose the following five specific aims: 1) To examine the impact of reduced TrkB receptor expression and/or signaling in phrenic motoneurons on functional recovery of rhythmic phrenic activity after SH; 2) To determine whether the continuing presence of neurotrophins (long-term effect) increases the relative expression of TrkB.FL in phrenic motoneurons after SH; 3) To determine changes in downstream pathways of TrkB.FL signaling in phrenic motoneurons after SH; 4) To determine whether time-dependent changes in TrkB signaling in phrenic motoneurons post-SH mediate the acute enhancing effect of intrathecal BDNF treatment on functional recovery during different behavioral conditions; and, 5) To determine whether functional recovery of rhythmic phrenic activity after SH is enhanced by increasing TrkB.FL expression in phrenic motoneurons using intrapleurally-administered gene transfer therapy. PUBLIC HEALTH RELEVANCE: Spinal cord injury is a devastating problem that affects about 500,000 people in the United States, with 11,000 new cases each year. The diaphragm muscle is the most important inspiratory muscle and it is paralyzed or seriously impaired in many cases of spinal cord injury. The proposed studies will provide important new information regarding the mechanisms underlying recovery of phrenic nerve activity and diaphragm function following spinal cord injury.